Reduce bgw_job_stat_history test flakiness

[fabriziomello]

Removed the unecessary start/stop background workers during the test execution to avoid canceling any ongoing job execution leading to non-deterministic status checks.

https://github.com/timescale/timescaledb/actions/runs/9491560936/job/26157244186

Disable-check: force-changelog-file

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.69%. Comparing base (59f50f2) to head (0ea36a5). Report is 210 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7030 +/- ## ========================================== + Coverage 80.06% 81.69% +1.62% ========================================== Files 190 199 +9 Lines 37181 36998 -183 Branches 9450 9672 +222 ========================================== + Hits 29770 30224 +454 + Misses 2997 2880 -117 + Partials 4414 3894 -520 ```

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